Loop signalling circuits have been developed in the past to increase the loop operating range by providing a replication of signalling functions particularly for long loop applications such as when the remote terminal (e.g. a telephone set or a PBX-private branch exchange) is more than about 5,000 meters from the central office. Such loop signalling circuits employed numerous option switches to provide for the different operating arrangements of the telephone equipment. For example, one switch was used to select either a loop start or a ground start mode of operation. Another switch was used to change the ringing detector response to provide for various duration ringing waveforms. Yet another switch was used to enable the detection of ringing signals from an ungrounded source at the central office. These loop signalling circuits were also inserted in series with the telephone lines to isolate some of the interference signals such as longitudinally induced power-line signals, and to boost and repeat the wanted signalling functions.
It is therefore desirable to have a single loop signalling circuit in which the signalling functions on the central office side of the telephone line are replicated on the remote terminal side and vice versa, regardless of the operating configuration (e.g. loop or ground start) of the telephone equipment. Such a circuit must provide all signalling functions. These include loop closure normally provided by a hookswitch, pulse dialling provided by a rotary switch, and ringing detection provided by the ringer at the remote terminal; and in addition, ring-ground detection, loop current detection, and dial pulse detection at the central office. In alternate applications such as with emergency telephones, ground start operation at the remote terminal and detection at the central office must also be replicated. Voice and data message signals are generally routed around the signalling circuit via a separate d-c isolated path.
A problem which is encountered in a circuit that operates under both loop start and ground start conditions, is the erroneous detection of a ground start condition in the presence of induced 60 Hz power line voltages. This problem is particularly acute when the circuit must function with both short and long loops that result in widely differing steady state current conditions.